1. Field of the invention
The present invention relates to a video disc system utilizing the CED or VHD capacitive pick-up system and, more particularly, to a pick-up resonator assembly for the readout of information carried in the video disc.
2. Description of the Prior Art
Generally, a resonator assembly 1 used in the CED capacitive pick-up system, as shown in FIG. 1, includes an oscillator 2 for producing, e.g., a 915 MHz sinusoidal wave signal, a resonator 3 with the resonant frequency of, e.g., 910 MHz for changing the operating point of the 915 MHz oscillator energy to 910 MHz, and a detector 7 for detecting the signal carried on 915 MHz signal. As shown in FIG. 1, the resonator 3 is connected through an input terminal 6 and a flylead 5 to an electrode provided on a stylus 4. The stylus electrode is generally formed by the deposition of titanium film or hafnium film on a diamond stylus base through the step of, e.g., sputtering. The stylus 4 is provided for the engagement with a video disc 13 which has a relatively fine spiral groove, defining a track, in which geometric indentations are formed to carry frequency modulated signals of picture and/or sound. During playback, the stylus 4 engages the groove as the disc 13 is rotated by a support turntable (not shown), producing capacitive variations between the stylus electrode 4 and the conductive disc 13. The stylus-disc capacitance is made part of the resonator 3. As the stylus-disc capacitance changes, the frequency of the resonant peak changes, as shown by the curves a, b, and c shown in FIG. 2, causing amplitude modulation of the 915 MHz signal passed through the circuit, as shown by the waveform 14 in FIG. 2. The amplitude-modulated 915 MHz signal is then demodulated by detector 7. Thus, the detector 7 produces from an output terminal 8 a frequency modulated signal that rises and falls in voltage as the surface of the disc 13 rises and falls under the stylus 4.
The output 8 of the resonator assembly 1 is connected to a preamplifier 10 including an amplifier 11 and an automatic frequency tuning (AFT) circuit 12. The amplifier 11 amplifies the frequency modulated signal obtained from the terminal 8. The AFT circuit 12 detects the d.c. component of the frequency modulated signal from the terminal 8, and compares the detected d.c. component with a predetermined d.c. level produced from a reference voltage generator provided in the AFT circuit 12. Then, the AFT circuit 12 produces an AFT voltage signal, which is a signal relative to the difference between the detected d.c. component and the predetermined d.c. level.
It is to be noted that the AFT circuit 12 is provided for correcting the change of resonant frequency due to the change of average capacitance between the stylus 4 and the disc 13 or for correcting the deviation of resonant frequency due to the uneven of the flylead 5 or stylus 4.
Referring to FIG. 3, there is shown a circuit diagram of one example of a resonator assembly 1 according to the prior art. The resonator assembly 1 shown includes a metallic casing 15 which is connected to ground. The metallic casing 15 has a separation wall 15a for separating the casing 15 into two cavities CA1 and CA2.
Accommodated in the left cavity CA1 is the oscillator 2 formed by capacitors C1 to C5, resistors R1 to R4, a transistor Q1 and a transmission line L1. As apparent to those skilled in the art, the oscillator employed is a base grounded Colpitts type. Also accommodated in the left cavity CA1 is a transmission line L2 located closely adjacent to the transmission line L1 for effecting the electromagnetic mutual coupling between the transmission lines L1 and L2.
Accommodated in the right cavity, i.e., resonator cavity CA2 are the resonator 3 formed by capacitors C7 and C8, choke coil L5, varactor diode (variable capacitance diode) D1, and tuned line L4, and the detector 7 formed by capacitors C9 and C10, diodes D2 and D3, and transmission line L6. Also accommodated in the right cavity CA2 is a transmission line L3 which extends through an opening formed in the separation wall 15a to the left cavity CA1 for the electric connection with the transmission line L2 via a capacitor C6. The transmission line L3 and the tuned line L4 are located adjacent to each other to effect the electromagnetic mutual coupling and, similarly, the tuned line L4 and the transmission line L6 are located adjacent to each other to effect the electromagnetic mutual coupling.
One end of the tuned line L4 extends outwardly from the cavity CA2 through an opening formed in the casing 15, and is connected with the terminal 6 which is in turn connected to the stylus electrode 4. Since the capacitance between the stylus electrode 4 and the disc 13 varies in a manner described above, the stylus electrode 4 and the disc 13 are shown by the symbol of a variable capacitor. The disc 13 is connected to ground.
The other end of the tuned line 14 is located inside the cavity CA2 and is connected with the casing 15 through capacitor C7 and varactor diode D1.
The prior art resonator assembly is disclosed, e.g., in a document "RCA SelectaVision VideoDisc SFT100 Player Technical Manual" first eddition 8048 V5 distributed to public in 1980, in Chapter four under a title "Video Disc Signal Retrieval".
Here, it is to be noted that the AFT voltage signal from the AFT circuit 12 is applied through the terminal 9, choke coil L5 to a junction 16 between the capacitor C7 and the varactor diode D1. Accordingly, the AFT voltage modulates the effective length of the tuned line L4.
It is also to be noted that according to the prior art resonator assembly described above, the capacitor C7 cuts d.c. component from the junction 16 and, at the same time, the capacitor defined by the stylus electrode 4 and the disc 13 also cuts d.c. component from the ground. Accordingly the tuned line L4 and the stylus electrode 4 are, from the view point of d.c. current, disconnected from any part of the circuit, even from the ground.
In operation, the disc 13 is first placed on the turntable and, then, a pickup arm carrying the stylus 4 is so moved as to effect the engagement between the stylus 4 and the grooves in the disc 3. During the movement of the pickup arm, the distance between the stylus 4 and the disc 13 becomes very small, causing a sudden increase of capacitance therebetween. The increase of capacitance between the stylus 4 and the disc 13 results in the shift of resonance frequency from 910 MHz to a lower frequency, e.g., shown by a broken curve a in FIG. 2. Accordingly, the AFT circuit 12 produces a AFT voltage signal to the terminal 9, resulting in the increase of voltage level at the junction 16. Thus, the voltage level along the tuned line L4 and that at the stylus electrode 4 increases relatively. Since the disc 13 is held to the ground level, there is a potential difference produced between the stylus electrode 4 and the disc 13.
In addition to above, the potential difference between the disc 13 and the stylus electrode 4 is produced by the electrostatic charge given to the stylus electrode 4 when the stylus electrode 4 is held closely adjacent to the disc 13 rotating at a high speed.
Such a potential difference results in spark discharge which gives rise to a damage to the stylus electrode 4 or to the surface of the disc 13.